The thermal spray process has been widely used to deposit coatings for industrial applications, including aerospace, automotive, petroleum and petrochemical, biomedical, etc. See Mateyja D., Plasma spraying of metallic and ceramic coatings. 1989, New York: John Wiley & Sons, the disclosure of which is hereby expressly incorporated by reference in its entirety. Thermal spray ceramic coatings are usually made from a powder feedstock. However, individual ultrafine particles (usually <5 μm) cannot be thermally sprayed using conventional powder feeding methods. A high carrier gas flow rate is required for the ultrafine particle penetration within the plasma jet. Such a cold gas flow rate will drastically perturb the plasma jet. See Fauchais, P., et al., Parameters controlling liquid plasma spraying: Solutions, sols, or suspensions. Journal of Thermal Spray Technology, 2008 17(1): p. 31-59, the disclosure of which is hereby expressly incorporated by reference in its entirety. On the other hand, these ultrafine particles would clog the hoses and fittings during delivery from the powder feeder to the thermal spray torch. See Lima, R. S. and B. R. Marple, Thermal spray coatings engineered from nanostructured ceramic agglomerated powders for structural, thermal barrier and biomedical applications: A review. Journal of Thermal Spray Technology, 2007 16(1): p. 40-63 and Chen, Z., et al., Air-plasma spraying colloidal solutions of nanosized ceramic powders. Journal of Materials Science, 2004 39(13): p. 4171-4178, the disclosure of each of these documents is hereby expressly incorporated by reference in its entirety.
Recently, a suspension plasma spray (SPS) process has been developed for the deposition of nanostructured coatings. See Chen, Z., et al., Air-plasma spraying colloidal solutions of nanosized ceramic powders. Journal of Materials Science, 2004 39(13): p. 4171-4178, Fauchais, P., et al., Suspension and solution plasma spraying of finely structured layers: potential application to SOFCs. Journal of Physics D-Applied Physics, 2007 40(8): p. 2394-2406, Burlacov, I., et al., Induction plasma-sprayed photocatalytically active titania coatings and their characterisation by micro-Raman spectroscopy. Surface & Coatings Technology, 2006 201(1-2): p. 255-264, Tomaszek, R., et al., Microstructural characterization of plasma sprayed TiO2 functional coating with gradient of crystal grain size. Surface & Coatings Technology, 2006 201(1-2): p. 45-56, Toma, F. L., et al., Nanostructured photocatalytic titania coatings formed by suspension plasma spraying. Journal of Thermal Spray Technology, 2006 15(4): p. 587-592, Berghaus, J. O., B. Marple, and C. Moreau, Suspension plasma spraying of nanostructured WC-12Co coatings. Journal of Thermal Spray Technology, 2006 15(4): p. 676-681, Fauchais, P., et al., Understanding of suspension DC plasma spraying of finely structured coatings for SOFC. Ieee Transactions on Plasma Science, 2005 33(2): p. 920-930, Wittmann-Teneze, K., et al., Nanostructured zirconia coatings processed by PROSOL deposition. Surface & Coatings Technology, 2008 202(18): p. 4349-4354, Chen, D. Y., E. H. Jordan, and M. Gell, Microstructure of Suspension Plasma Spray and Air Plasma Spray Al2O3-ZrO2 Composite Coatings. Journal of Thermal Spray Technology, 2009 18(3): p. 421-426, and Chen, D. Y., E. H. Jordan, and M. Gell, Suspension plasma sprayed composite coating using amorphous powder feedstock. Applied Surface Science, 2009 255(11): p. 5935-5938, the disclosure of each of these documents is hereby expressly incorporated by reference in its entirety. In SPS, the ultrafine or nano sized particles are dispersed in a liquid medium such as water or ethanol to form a suspension and then the suspension is injected into the plasma torch. The suspension droplets will undergo liquid evaporation, particles melting process in the plasma jet and form the coatings upon impacting the substrate. Chen, D. Y., E. H. Jordan, and M. Gell, Microstructure of Suspension Plasma Spray and Air Plasma Spray Al2O3—ZrO2 Composite Coatings. Journal of Thermal Spray Technology, 2009 18(3): p. 421-426 and Chen, D. Y., E. H. Jordan, and M. Gell, Suspension plasma sprayed composite coating using amorphous powder feedstock. Applied Surface Science, 2009 255(11): p. 5935-5938, used the molecularly mixed amorphous powder as feedstock and prepared phase homogeneously distributed ceramic coatings using the suspension plasma spray process. Waldbillig, D. and O. Kesler, The effect of solids and dispersant loadings on the suspension viscosities and deposition rates of suspension plasma sprayed YSZ coatings. Surface & Coatings Technology, 2009 203(15): p. 2098-2101, the disclosure of which is hereby expressly incorporated by reference in its entirety, studied the effect of solids and dispersant loading on the deposition rate of suspension plasma sprayed YSZ coatings. In the reported suspension plasma spraying process, home-made delivery systems with a peristaltic pump were used to deliver the suspension to the plasma jet.
While liquid jet injection into plasma was well observed and studied (see P. Fauchais and G. Montavon, Latest Developments in Suspension and Liquid Precursor Thermal Spraying. Thermal Spray 2009: Proceedings of the ITSC, 2009, ASM International: p. 136-149, R. Etchart-Salas, V. Rat, J. F. Coudert and P. Fauchaus, Parameters controlling properties of coatings sprayed by suspension plasma spraying. Procedings of the ITSC, 2008: p. 506-511, C. Marchand, C. Chazelas, G Mariaux and A. Vardelle, Liquid precursor plasma spraying: observation of liquid feedstock break-up. Procedings of the ITSC, 2008: p. 512-516, and R. Vassen, H. Kassner, G. Mauer and D. Stover, Suspension plasma spraying: Process Development and Applications. Thermal Spray 2009: Proceedings of the ITSC, 2009, ASM International: p. 162-167, the disclosure of each of these documents is hereby expressly incorporated by reference in its entirety), the propensity of suspensions to clog lines and injector orifice gained notoriety among researches and practitioners alike, and rendered the SPS process as highly unreliable and impractical. Combined with a high cost of sub-micron and nano-sized feedstock, any significant loss of material due to line flush or non-stop running further diminishes ROI and prospects of practical SPS process implementation. To make stable and robust coating deposition, there is a great need for the development of commercially available liquid delivery system for the suspension/solution thermal spraying.